The usual heater type cathode, as shown in FIG. 1, includes a cathode material 1 as a source of electrons, a metal base 2 for spreading the cathode material thereon, a sleeve 3 for supporting the base 2 and for enclosing a heater 5, and a holder 4 for supporting the sleeve 3. The above mentioned components are separately manufactured, assembled by spot welding, and the cathode material 1 is spread on the surface of the base 2 by spraying.
In such a cathode, the heat energy is transmitted from the heater 5 to the metal 2 and to the cathode material 1 forming the surface layer of the base 2 to cause thermal electron emission. To obtain a desired thermal electron emission, the transmission efficiency of heat from the heater 5 to the base 2 has to be maximized.
However, a heater type cathode employs an indirect heating method and, consequently, increases in thermal efficiency are limited. In such a heater type cathode, there are many causes for heat loss such as the thermal resistance between the sleeve and the base, heat radiation resistance, and leakage of the radiated heat between the sleeve and the heater. Particularly, the thermal resistance and the radiated heat leakage are large factors reducing thermal efficiency.
The thermal resistance is further increased because the base metal and the sleeve are spot-welded at a plurality of spots, thereby leaving small gaps other than at the welded spots. Meanwhile, the radiated heat leakage occurs through the lower opening of the sleeve. An experiment demonstrated that the radiated heat leakage accounts for the greater part of the total heat loss.
Such heat losses, increase the time required for heating up the cathode material to the operating temperature through the heat energy of the heater. If this time increase is to be prevented, the amount of the current supplied to the heater has to be raised, increasing power consumption, but there are many restrictions against increasing consumption of electric power.